Method of making silica glass honeycomb structure from silica soot extrusion

ABSTRACT

The present invention describes an extrusion process for manufacturing a glass honeycomb structure having a variety of shapes and sizes depending on its ultimate application. Unlike prior art honeycomb structures made from ceramics, the inventive glass honeycomb can be readily viscously bent and/or redrawn. Furthermore, the inventive honeycomb structure is lightweight, yet able to support heavy loads on its end faces. Therefore, the inventive honeycomb can be used as a light-weight support for such objects as mirrors. These honeycombs can be used singularly or in aggregates to provide such support. Embodiments are described wherein the mass of the honeycomb is further reduced by removing select portions of the honeycomb without deleteriously impacting its ability for load bearing.

This application claims the benefit of U.S. Provisional Application Ser.No. 60/120,629, filed Feb. 18, 1999 entitled Silica Glass HoneycombStructure From Silica Soot Extrusion, by C. Charles Yu, John F. Wight,Gitimoy Kar and Kenneth E. Hrdina

RELATED APPLICATIONS

This application is related to the following, commonly assigned,co-pending United States patent applications: U.S. Ser. No. 09/211,379filed Dec. 19, 1997 by D. St. Julien et al. entitled MANUFACTURE OFCELLULAR HONEYCOMB STRUCTURES now U.S. Pat. No. 6,299,958; U.S.application, Ser. No. 09/299,766 filed Apr. 26, 1999 by Borrelli et al.entitled Redrawn Capillary Imaging Reservoir; and U.S. Ser. No.09/360,672, filed Jul. 30, 1998 by Borrelli et al. entitled METHOD OFFABRICATING PHOTONIC STRUCTURES now U.S. Pat. No. 6,260,388; and U.S.application, Ser. No. 09/300,121 filed Apr. 27, 1999 by Borrelli et al.entitled REDRAWN CAPILLARY IMAGING RESERVOIR, now U.S. Pat. No.6,350,618.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for making vitreous honeycombstructures and, more particularly, for making high purity vitreoushoneycomb structures from silica soot by extrusion. These structures canbe used by themselves or as preforms for consecutive processes such asviscous reforming operations.

While common structures made of glass have become inseparable from ourdaily lives, the unique properties of glass also allow its use incomplex structures for high-technology specialty applications. Honeycombstructures comprising glass belong in the latter category. Glasshoneycomb can be made with an array of channels and indeed have beenmade by other processes usually requiring fusing many individual glasstubes together. Structures made in such a manner are typically limitedin channel diameter and homogeneity due to the fusing process.Structures of this type have also been made of other materials, butglass offers a combination of unique properties that allow such astructure to be hot drawn down and used in novel applications andtechnologies, especially where high surface area is required. An exampleof such a utility would be to facilitate or catalyze chemical reactions.Additional benefits derive from the high purity and/or high clarity andtransparency that can be obtained from a glass honeycomb article. Ahoneycomb structure composed of glass is, therefore, an ideal articlefor supporting reactions requiring the initiation by actinic light.

Furthermore, because of the cell-like structure, glass honeycombstructure is extremely strong along the channel axis and yet issignificantly lighter in weight than solid bulk glass. It is thus idealfor use as a support for such items as mirrors and the like by forming asandwich construction (ref. CELLULAR SOLIDS, STRUCTURES, AND PROPERTIES,2nd ed., Lorna J. Gibson and Michael F. Ashby, 1997). Glass honeycombmaterials can have a significant benefit where weight is an issue as,for example, extraterrestrial payload. In one example, it is especiallybeneficial for supporting the reflecting surface of a mirror. Thehoneycomb support material additionally can be of similar low thermalexpansion coefficient to the mirror material in order to preventdistortion or breakage due to thermal stress.

Silica soot is a by-product of the high purity fused silica glass makingprocess. Until now, it has been considered a waste material that istypically discarded even though it is essentially pure silicon dioxide.The increasing demand for high purity fused silica exacerbates thiswaste problem. Therefore, there is a strong desire to reduce thiswastestream both from an ecological as well as a financial perspective.Most advantageous would be to find a productive, commercial applicationfor the material.

2. Prior Art

Conventional processes have been used to create glass honeycombstructures, but these differ considerably from the inventive process.The prior art approaches to manufacturing this type of glass honeycombarticle are either to fuse individual hollow fibers or tubes together orto machine out a solid piece of glass to form a multi-channelledarticle.

These prior art processes are problematic for several reasons. Firstly,it is difficult to fuse multiple hollow fibers (i.e., fine capillarytubes) to form a multi-channelled article which can then optionally behot-drawn down and rebundled again and again into a progressively finerand finer array of hollow channels. Secondly, it is difficult toassemble and fuse multiple hollow tubes uniformly into a perfecthoneycomb structure. Thirdly, the diameter of the individual hollowfibers that can be easily handled limits the number of tubes in thefirst bundle towards making the honeycomb structure, because there is apractical limit to the diameter of the assembly that can be uniformlyhot-drawn down. Lastly, it is extremely expensive and time consuming tomachine a multitude of deep channels into a glass object.

Ceramic honeycomb structures such as Celcor® (a cordierite honeycombstructure used commercially as a substrate for automotive catalyticconverters) have been paste-extruded from particulate material, but theresulting honeycomb article is not transparent to light, reducing itsutility. In addition, such honeycomb article is crystalline in nature,preventing it from post forming such as hot-drawn down. Further, theparticle size of the raw material used in the Celcor® process isapproximately two orders of magnitude larger than the soot used in thepresent invention. The particle size can significantly affect theminimum web thickness for an extrudable honeycomb structure by directextrusion.

It is therefore an objective of the present invention to provide acommercial application for high purity silica soot.

It is therefore another objective of the present invention to provide acommercial application for silica soot.

It is another object of the invention to provide a paste-extrusion andsintering process for the conversion of the silica soot into a glassarticle.

It is further an object of the invention to provide a glass honeycombstructure having high optical clarity and/or high UV transmission,coupled with good mechanical strength, and excellent thermal stability.

It is yet another object of the invention to utilize a glass honeycombstructure in such technologies as filtration, water purification,membrane reactors, flow controllers, bio-reactors, structuraldielectric, and structural supports.

SUMMARY OF THE INVENTION

The current invention is designed to address the above-mentioned objectsand prior art deficiencies. In particular, a process is disclosed forconverting a silica containing vitreous powder, specifically a highpurity fused silica (HPFS®) or a doped silica soot, into a glasshoneycomb article. This is the first known process utilizing fusedsilica soot to generate such articles which themselves possess uniqueand advantageous properties.

Glass honeycomb structures can be made by adapting the conventionalpractice of honeycomb paste-extrusion to using vitreous powder as thestarting material. Of special interest is the high silica content glasshoneycomb structures made of high purity fused silica soot, and sootderived from silica and other metals, for example silica soot containingup to 9% titanium. High purity silica is defined as essentially puresilicon dioxide, trace materials may be present due to specific processor isolation techniques but these trace materials are consideredcontaminants adding no beneficial properties to the pure silicondioxide.

BRIEF DESCRIPTION OF THE DRAWINGS

A complete understanding of the present invention may be obtained byreference to the accompanying drawings, when considered in conjunctionwith the subsequent detailed description, in which:

FIG. 1 schematically illustrates a side cross-sectional view of a glasshoneycomb article in accordance with the present invention;

FIG. 2 schematically illustrates a top view of the frontal end of aglass honeycomb article in accordance with the present invention;

FIGS. 3a-3 d schematically illustrate top views of four frontal ends ofa glass honeycomb articles to depict preferred cross-sectional channelshapes;

FIG. 4 schematically illustrates a side cross-sectional view of a glasshoneycomb article in accordance with the present invention, the lowerportion of which has been redrawn;

FIG. 5 schematically illustrates a glass honeycomb article in a U-shapedconfiguration with one end of the article having a constricted opening;

FIG. 6 is a schematic representation of sectioning the honeycomb glassarticle of FIG. 1 into a multitude of essentially identical, finitelength honeycomb articles, wherein the longitudinal length issignificantly longer than the cross-sectional width;

FIG. 7 is a schematic representation of the honeycomb glass articlesectional being processed into a multitude of essentially identical,finite length honeycomb articles, wherein the longitudinal length issignificantly shorter than the cross-section width; and

FIG. 8 schematically depicts a bellmouth lightweight glass honeycombarticle wherein the frontal ends have a larger surface area than thecolumnar cross-sectional surface area and said frontal end supporting aweight-bearing member.

For purposes of clarity and brevity, like elements and components willbear the same designations and numbering throughout the figures.

By way of definition, the term “honeycomb structure” as used in thespecification, describes an extruded glass article 100 having twoopposing faces 10 a and 10 b, outer longitudinal surface 20 of length L,and a matrix of cell walls 40 defining an array of channels, cells orthrough-holes 30, wherein each opposing face has a correspondingcross-sectional surface area and average cross-sectional diameter (D_(a)and D_(b) respectively), and each channel traverses, along alongitudinal axis, from the first face 10 a to the second face 10 b. Theopposing faces may have identical cross-sectional surface areas in whichcase the channels will traverse the honeycomb article parallel to eachother. The channels may be disposed randomly or at a fixed distance fromeach other. This distance is defined by the cell wall thickness, t. Thechannels also will have a cross-sectional shape and size defined by thecell wall. All closed shapes (e.g. circles, ellipses, triangles,squares, rectangles, hexagons) are allowed. The individual channels canbe of all the same shape or mixtures thereof. The cross-sectional size,d, of the channel can be either fixed for all channels or vary withinthe honeycomb article. The honeycomb article itself can be in a lineardesign, in which case the first and second opposing faces are parallelto each other, or bent along the longitudinal axis.

DETAILED DESCRIPTION OF THE INVENTION

The extrusion process as designed for this invention is similar to thatused to make Cordierite Celcor™, for examples see U.S. Pat. No.3,790,654 (Bagley), U.S. Pat. No. 3,905,743 (Bagley), U.S. Pat. No.4,551,295 (Gardner et al.), 4,902,216 (Cunningham), and U.S. Pat. No.5,602,197 (Johnson et al.). To avoid repetition, these publiclyavailable documents are incorporated herein in their entirety andreference is made thereto. Except as otherwise indicated hereinafter,the present invention does not contemplate any substantial change inpreviously disclosed methods of these documents.

Unlike other production processes, the formation of high purity fusedsilica (HPFS®) soot is generated by a unique flame hydrolysis or flamecombustion process under specifically designed environmental conditions.High purity silicon containing chemical is introduced into anoxygen-hydrocarbon, or oxygen-hydrogen flame, to generate silicaintermediates in an insulated enclosure which is maintained attemperatures above 1600° C. The silica intermediates include “seeds” ofsolid silicon dioxide in the nanometer size range, gaseous siliconmonoxide, and other intermediate silicon containing compounds from theflame hydrolysis or flame combustion reactions.

The insulating enclosure is designed in such a way that the silicaintermediates experience prolonged residence time under high temperature(>1600° C.) within the enclosure, during which the solid silicon dioxide“seeds” grow and sinter simultaneously to generate larger particlesbefore exiting the enclosure.

In this regard, silica soot possesses several unique properties thatmake it a potentially useful raw material. For example, silica sootcomprises high purity, dense, spherical particles having a diameterbetween about 0.05 and 0.4 micron with an average size of about 0.2micron. Titanium containing silica soot is a by-product of the ultra lowexpansion (ULE™) glass making process. It has similar characteristics tohigh purity fused silica soot except for its composition. Both sootpowders have a broad particle size distribution conducive for highparticle packing efficiency. Packing efficiencies up to 80% are possiblebut practical limitations, especially during the extrusion process,limit the upper value. Preferred values for packing efficiency arebetween about 45% and 70%, and most preferred 65%.

One of the differences between the present invention and prior artprocesses is that silica glass soot is used as the starting particulateand that low-sodium containing processing additives are desirable in thepresent invention for applications pertinent to UV transmission. Forexample, stearic acid and/or oleic acid are preferred to sodium stearateas a lubricant for a soot derived paste formulation. Elvanol® has lesssodium ash than Methocel®, and therefore is more desirable. However,chemical cleaning of the extrudate before its final sintering willsignificantly reduce sodium to the degree that the extrudate is purerthan the starting powder.

Another difference between the present invention and prior art processesis that the solids loading of the soot paste is 60 to 63 volume percent(volume of the soot divided by the volume of the paste×100%), which is ahigher loading than the standard 50 volume percent solids loading of thepaste for making cordierite Celcor™. This is attributed to the sootparticle's essentially spherical shape and its size distribution. Twoexample recipes, one using Methocel® and the other using Evanol® as abinder, for 30 making the glass honeycomb structures are given below.Both batches can be ram extruded (3.5″ diameter barrel) through a 1″diameter 400/8 Celcor® die (a die for extruding a square channelgrid-array with 400 channels per square inch and a channel wallthickness of 0.008 inches). For further details, refer to Bagley, U.S.Pat. No. 3,790,654 and Cunningham, U.S. Pat. No. 4,902,216. For otherdiameters and for auger extruders, the water level in the formulationsbelow can be adjusted accordingly.

Paste of Formula 1: Silica soot 1000.0 gm Dow Methocel ® F40M 31.8 gmStearic Acid (fine powder) 4.0 gm Deionized water 275.0 gm Paste ofFormula 2: Silica soot 1000.0 gm DuPont Evanol ® 50-42 28.7 gm StearicAcid (fine powder) 3.7 gm Deionized water 254.1 gm

Formulae 1 and 2 use deionized water as the solvent to prepare thestarting paste. However, also useful in the present invention areaqueous organic solvent mixtures. Suitable organic solvents are thelower alcohols, ketones, amides, and esters. These solvents must besufficiently soluble in water to provide a homogeneous mixture. Theratio of water to solvent can vary from 95:5 to 5:95. Most preferredsolvents are ethanol, acetone, methyl ethyl ketone,N,N-dimethylformamide, and ethyl acetate (see U.S. Pat. No. 5,458,834,Faber et al.).

For the Methocel® recipe, the dry ingredients are turbomixed together.Water is then added as the powder mix is mulled to form a paste. For theEvanol® recipe, a hot polymer solution is made by adding the polymer andthe stearic acid to the hot water. This is cooled and added to the sootduring mulling. The discharged chunks are spaghettied three times. Therubbery paste spaghetti is then pushed through a 400/8 Celcor® die(refer to U.S. Pat. Nos. 3,790,654 and 4,902,216 disclosed hereinabove)with an 8 mil shim gap and a knife-edged mask. The extruded honeycombstructure is first dielectrically dried to generate greenware, and thendebinded with up to a two day burnout with a maximum temperature of 900°C. to generate the brownware. Brownware is unsintered debindedgreenware. The following is a typical debinding (pyrolysis) schedule togenerate the brownware from the greenware piece:

20-200° C. @ 50° C./hr

200-600° C. 20° C./hr

600-900° C. 50° C./hr

900° C. for a six hour hold

900-600° C. 50° C./hr cooling

600-20° C. @ cooling without power.

Sintering initiates at the final stage of debinding, when thetemperature exceeds 850° C. As a result, the honeycomb structure afterdebinding has enough mechanical strength to be handled for subsequenttreatment.

When high purity fused silica (HPFS®) soot is used as a raw material,the optional step of chemical cleaning is performed. In this step,intermediary brownware article is heated to a temperature between 850°and 1300° C. for a period greater than about 20 minutes in a chlorine orfluorine containing atmosphere for removing beta-OH and tracecontaminants of alkali (e.g. sodium), and alkaline earth elements andiron from die wear. When such chemical cleaning is desired, it isperformed prior to the sintering step.

The soot honeycomb substrates can be sintered to various degrees toobtain structures that range from open porosity to optical clarity. Thesintered glass honeycomb can also be viscously formed into a variety ofshapes, examples of which are disclosed in the supra, (U.S. patentapplication Ser. No. 09/360,672 for the first glass Celcor fiber, nowU.S. Pat. No. 6,360,388, and Ser. No. 09/300,121 (attorney docketBorrelli 80A) for the first glass Celcor funnel, now U.S. Pat. No.6,350,318).

Transparent HPFS® honeycomb structures have been obtained by heattreating the brownware piece to temperatures up to 1760° C. in a vacuum.A typical firing schedule to sinter the glass structure follows:

a) load specimen into furnace; draw a vacuum at room temperature,

b) heat furnace at about a 50° C./minute rate to 1000° C.,

c) hold at 1000° C. for 5-15 minutes,

d) increase temperature at 10-15° C./minute to 1650° C.,

e) increase temperature at 2° C./minute to 1760° C.,

f) hold at 1760° C. for 5-15 minutes,

g) backfill furnace with argon gas, and

h) cut off power to furnace and allow article to cool at furnace rate toroom temperature.

The vacuum firing procedure given above is for the purpose of anillustrating example. The firing can also be carried out effectivelyunder air or inert gas atmosphere such as nitrogen, helium, argon,carbon dioxide, forming gas, and the like, and mixtures thereof. The gasatmosphere can be applied at less than, but preferably equal to, orgreater than ambient pressure. Different heating rates and isothermalholding times will be employed when gas atmospheres are employed. Thedetermination of the appropriate conditions is readily determined byexperimentation but not excessive experimentation.

Although it is preferred that the glass honeycomb article produced bythe above inventive process be transparent, it is within the scope ofthis invention that the article can also be translucent or opaque.Furthermore, the glass honeycomb article can either be colorless or havea colored tint. Coloration can be achieved by incorporating traceamounts of known glass metal oxide dopants such as cobalt oxide, nickeloxide, or the like.

The glass articles derived from the extruding process using silica sootas a starting material possess unique characteristics and propertiescompared to alternative processes. As mentioned supra, glass honeycombstructures can be made by fusing together individual tubes of glass.This crude process is both cumbersome and difficult to control in termsof product uniformity. Furthermore, it is very much limited to thediameter range for the individual channels. Below certain sizes, theprocess becomes untenable.

Glass honeycomb can also be made from an extrusion process using groundglass powder as a starting material (see for example, U.S. patentapplication Ser. No. 09/360,672 now U.S. Pat. No. 6,260,388, and Ser.No. 09/300,121 now U.S. Pat. No. 6,350,318 supra). In these situations,some of the disadvantages of the fusing process are alleviated but otherproblems still remain. Specifically, problems of uniformity are greatlydiminished compared to the fusing process. However, the powders aretypically on the order of about 5 to about 50 micron in diameter. Theimpact of particle size is its relationship to thickness of channelwalls within the honeycomb. For good extrudablility and mechanicalstrength, cell walls are typically at least 5 to 10 particles inthickness. Therefore, the larger the particle size, the greater theminimum thickness of the cell wall. Since, in the present inventionparticle size is on the order of 10 to 100 times smaller than theparticle size used in conventional glass powders, glass articles withcorrespondingly thinner walls are extrudably achievable by the presentinvention. Conventional glass powder has contamination from the melttank refractories and grinding media. These stones do not melt or drawdown. And, their size limits the extent to which web wall thicks can bedrawn down. Soot does not have “stones”.

As will be seen hereinbelow, wall thickness is directly related to openfrontal area (OFA) which essentially is the ratio of channelcross-sectional area to total honeycomb cross-sectional area. Thethinner the wall thickness, the higher the OFA and therefore the lighterthe weight of the honeycomb. The weight to volume ratio of the honeycombarticle is significant when the glass honeycomb is being considered foruse as a light weight support.

Wall thickness is also an important parameter for applications involvingdiffusion of materials through cell walls. Where this characteristic isdesirable, such as high efficiency filtration systems or membranereactors, the greater the diffusion rate, the more efficient therespective process. The thinner cell walls of the current inventionassist in designing superior articles of this type. The presentinventive formulation and process can extrude glass honeycomb greenware(prior to hot draw or sintering) with channel walls of a minimum ofabout 10 micron, a preferred minimum of about 40 micron, and a mostpreferred minimum of about 100 micron.

Furthermore, high purity fused silica, and the soot byproduct used inthe present invention, offer a significant advantage over the powdersilicates used in alternative, powder processes. The high purity fusedsilica of the present invention is of such purity, having extremely lowlevels of alkali and alkaline metals, iron and copper (e.g., below50-100 ppm), that very high UV transmission is obtained. Forapplications involving absorption of actinic radiation, specificallyultraviolet radiation, this characteristic is extremely useful. Examplesof such applications include, bio-reactors, and in-situ waterpurification.

High purity fused silica soot, as used in the current invention, alsoprovides another significant advantage over other materials, namely; ahigh softening point which is typically greater than 1100° C. Thisallows honeycomb devices made from this material to be used in hightemperature applications.

The soot can be further composed of glass-forming metal oxides (i.e.,Pyrex®), including but not limited to, aluminum oxide, phosphorousoxide, boron oxide. Preferably the amount of such metal oxides is below50%, the remainder being silicon dioxide. Where other properties of thehoneycomb are desired dopants can also be employed in the soot, forinstance, dopants to modify refractive index (e.g. lead oxide) or toimpart color (e.g. cobalt oxide) can be used in percentages up to about10%.

In one embodiment of the invention, the extruded glass honeycomb articlecan optionally be further processed to narrow the overall cross-sectionof the article. This can be performed by any process that viscouslydraws down the article in a longitudinal direction. Typically, this isperformed by redrawing the article in a heated environment to atemperature high enough to lower the viscosity of the glass to a pointthat the glass honeycomb article is soft enough to be viscously drawndown. In this manner the individual channel cross-sections are reducedin size (see U.S. patent application Ser. No. 09/300,121 to Borrelli nowU.S. Pat. No. 6,350,618). For HPFS® honeycomb structures, thetemperature required to render the redrawn feasible is in the rangeabove 1800° C.

In another embodiment of the invention, elongation is performed only toone end of the article, thereby producing one constricted end havingnarrowed channel cross-sections. An article of this type can be used tosimultaneously supply the same or multiple solutions to a very smallreceptive area. Solutions can be provided continually or intermittentlyas required by the specific application.

In other embodiments of the invention, the glass honeycomb article maybe linear along its longitudinal axis or it may be curved. In onepreferred embodiment, the longitudinal axis is curved into a U-shapedarticle. The ends of the U-shaped article can be either the same ordifferent cross-sectional areas depending if the article is alsoredrawn.

In still another embodiment of the invention, the glass honeycombarticle can be redrawn while torsional forces are being applied. Thisproduces a twisting of the channels around the longitudinal axis andprovides a helical honeycomb article (see U.S. Pat. No. 5,633,066, Lippet al.).

In yet another embodiment of the invention, the glass honeycomb articlecan be sectioned along its longitudinal axis after optionally beingredrawn. In one preferred embodiment, after cutting, the distance alongthe longitudinal axis is significantly greater than the cross-sectionaldiameter of the honeycomb. In this manner, a tube is created havingmultiple channels. In a second preferred embodiment, after cutting, thedistance along the longitudinal axis is equal to or shorter than thecross-sectional diameter of the honeycomb, thus creating a honeycombedplate or screen article.

In still another embodiment of the invention, one or both ends of thehoneycomb article can be capped or sealed with a capping material 50.This material can be composed of any known solid substance including,but not limited to, glass, metals, organic polymers and resins. Thesematerials may be colored or colorless; opaque, translucent ortransparent; and they may have any thickness. In another preferredembodiment, the thickness or the refractive index of the cappingmaterial is not uniform and is designed to function as a lens forfocusing or diverging transmitted radiation. Furthermore, the cappingmaterial may be permeable to other materials such as liquids or gases.

The channels in any of the above-mentioned embodiments can be filled orpartially filled or coated with a material. This material may be asolid, liquid, gas, or combinations thereof. Specifically contemplatedis filling less than all channels.

The structure and cross-sectional shape of the hollow channels 30depends on the die used during the extrusion process. It is within thescope of this invention that channel structures possess across-sectional shape defined by a closed wall 40. These shapes include,but are not limited to, circles, ellipses, regular polygons, andirregular polygons having acute or obtuse angles with equal or unequalsides. These shapes may or may not have a point or plane of symmetry.Preferred shapes include squares, triangles, hexagons, and circles. Thehollow channels can be in a repetitive pattern within the article orrandomly placed along the cross-sectional surface. It is within thescope of the invention that one or more shapes may be formed within thecross-sectional area of the honeycomb. Furthermore, the cross-sectionalsize of the channels need not be uniform throughout the honeycombcross-section.

In one preferred embodiment, the channels would decrease in size as thedistance increases from the center of the cross-sectioned honeycomb.

The thickness, t, of the walls separating the channels can be constantfrom one channel to another, or they may vary in thickness between onechannel and another. Depending on the application of the honeycombarticle, either design is acceptable.

Typical characteristics of the extruded unsintered glass honeycombarticle prior to redrawing are, for example:

commercial cell density (CD) range: 16-900 cells per square inch;

commercial open frontal area (OFA) range: 0.55-0.87;

commercial substrate diameter (D) range: 0.25-12 inches;

commercial channel wall thickness (WT) that can be calculated from theequation: OFA=((CD^(−0.5)−t)/CD^(−0.5))²=(1−t(CD)^(0.5))².

The above ranges after firing change because of sintering shrinkage.Shrinkage depends on solids loading and the degree of sintering, butvalues between 13-20% isotropic linear shrinkage are typical.

Due to the unique combination of vitreous solid properties discussedsupra and the flexibility of the paste-extrusion process, the resultingfused silica honeycomb can find many applications outlined hereinbelow.

Bio-reactor: Silica surface is known to be bio-active. The cells of thehoneycomb structure can be used for culturing. It is also known thatmetabolism of certain microorganisms can be greatly enhanced with lowintensity light radiation at certain wavelengths (mostly in the UVregion). By feeding nutrient up-stream, while at the same time radiatingthe microorganism grown on the cell wall (by taking advantage of thehigh UV transmission of the HPFS), the transparent silica honeycombstructure can be used as a bio-reactor to generate useful bio-materials.

Capillary Flow Controller: Capillary tubes can restrict the flow offluid passing through them. They can regulate the flow at a ratherconstant rate for multiple orifices. Either through cold reduction ofgreen silica honeycomb structure (a process involving wax refill andreduction extrusion; see U.S. patent application, Ser. No. 09/211,379,attorney docket number St. Julien 15, now U.S. Pat. No. 6,299,958)followed by sintering, or through hot reduction by a hot drawn down,ultrahigh cell density with capillary channel can be achieved. Theefficiency of this honeycomb capillary flow controller is maximizedbecause of the ultra high packing density of the capillary channels.Excellent chemical and thermal stability of the high purity silica isadvantageous for such an application.

High Efficiency Filtration System: Partially sintered silica particlecompact has a 3-D interconnecting pore structure. Due to the small sizeand narrow size distribution of the soot particle, the resultingmicrostructure will consist of a 3-D porous network with passage in themiddle to upper nanometer range and narrow range and narrow sizedistribution. This is ideal for ultrafiltration applications. Withviscous sintering being the dominant mechanism, the size of the porouspassage can be well controlled by heat treatment temperatures between1050-1400° C. without sacrificing porosity. As a result, the partiallysintered silica honeycomb structure with controlled porous passage canbe used for filtration by selective channeling through the honeycombcells. High operating efficiency comes from the unique combination ofhigh cell density, thin cell wall, sharp passage cut-off, and highporosity. Chemical and thermal stability of high purity silica isadvantageous for such application.

Membrane Reactor: Similar to the filtration system but differing in onerespect, the membrane reactor is an active system. By loading catalystinto the 3-D porous network and selectively channeling differentreactant fluid through the honeycomb cells, kinetics of the chemicalreaction can be enhanced and equilibrium modified. As result, orders ofmagnitude increases in reaction rate and product yield can be achieved.Again, chemical and thermal stability of the silica is an addedadvantage.

In-situ Water Treatment: This application also utilizes the high UVtransmission properties of HPFS. Hazardous bacteria and virus can beeliminated by flowing contaminated water through the honeycomb structurewhile applying high intensity UV radiation from outside the honeycombstructure.

Microlens Array: Infiltrate the HPFS honeycomb cell with molten glass ofhigh refractive index, then immediately lower the temperature to therange where the reaction between the high refractive index glass and theHPFS (by diffusion mechanism) can be safely controlled. At anappropriate temperature with proper exposure times, it is possible togenerate a unique glass body consisting of a periodic array of parallelcolumns. The refractive index (RI) profile within individual columnswill be radially parabolic. The glass body can then be redrawn to reducethe size of these special index columns and the space between them.After being redrawn, the glass can be sliced into thin disks consistingof a microlens array. Selected lenses in the array can be masked togenerate patterns.

Photonic Band Gap Structure: It is possible to create an HPFS structurewith a periodicity in micron size range through a single stage or twostage reduction process. The single stage approach increases celldensity simply by hot redraw. The two stage approach starts with a coldreduction of green silica honeycomb structure (a process involving waxrefill and reduction extrusion; see U.S. patent application, Ser. No.09/211,379, now U.S. Pat. No. 6,299,958, followed by sintering, thenfinally through hot reduction by hot redraw. The orderly arrangement oftwo media (in this case, silica and air) with different RI gives rise toa photonic band gap as the periodicity approaches micron scale; see U.S.patent application, Ser. No. 09/360,672, filed Jul. 30, 1998, now U.S.Pat. No. 6,260,358. Patterned defects can be created by modifying theoriginal honeycomb cell structure to produce interesting opticaleffects.

High Temperature Dielectric Material: Many materials, includingpolyurethane foam, polystyrene foam, and foamed glass have been used asdielectric materials in applications such as specialty antennae andLundberg lenses. For high energy density and high temperatureapplications, organic polymeric materials can no longer performadequately. Different applications may require different dielectricconstant profiles within the material. For example, the Lundberg lensrequire the profile of highest dielectric constant at the core anddecreasing dielectric constant radially outward, with a concentricsymmetry. The dielectric constant is controlled by altering the hollowstructure of the material, a difficult task to achieve with precision inthe foamed structure, especially for high melting glass. Alteration ofthe dielectric constant can be readily achieved by either of twodesigns. The first would provide ever larger cross-sections for thehollow channels, as the distance increases from the center of theantenna. The second design provides channels of all the samecross-section. The number of such channels per unit area would increaseas the distance increases from the center of the lens.

The extrusion process for preparing honeycomb glass articles is suitedfor economically preparing such antennae and lenses with the addedbenefit that the composition profile can be manipulated through thefeeding process to the extrusion die, in order to gain additionalcontrol over the dielectric profile. Profile manipulation is notfeasible for foam structures.

Light Weight Structural Support: The honeycomb glass device disclosedsupra can also be used for a structural support, since the cellulardesign provides a sufficiently rigid and strong object to sustain largeforces placed along the cellular axis. Although the honeycomb glass isphysically durable and strong, it is also very light in weight. Thismakes it ideal for use as a structural support for massive objects,especially such articles as mirrors. Prior art designs for mirrorsessentially utilized a glass member machined to a highly polishedsurface. The mirror, being essentially of solid bulk glass, wastypically extremely heavy. Another conventional approach to creating asupport was to fuse pieces of glass together into a light weight core.Both means are expensive and time consuming.

If such articles were meant for extraterrestrial destinations then thebulk weight of the article became a limiting factor in its design. Thecurrent invention allows for a very light weight support made ofhoneycomb glass. Affixed or capped to one end of the support would bethe mirror element 50. Another feature of the design is that thecoefficient of expansion for the support can be designed to be equal tothat of the mirror element so that thermal changes would not causedistortions or physical cracking of the mirror element. For this reason,ceramic materials that previously could be manufactured in a honeycombdesign were not useful, since their coefficient of expansion was verydifferent from that of the glass mirror element. However, conventionalextrusion techniques for ceramics can be modified to prepare the glass,light weight support as discussed above.

A mismatch in the coefficient of thermal expansion (CTE) between themirror blank and the core results in permanent stresses between thecomponents during the bonding operation. It is desirable to minimize thestresses to reduce or eliminate the likelihood of cracks or long termcreep of the components. A second desirable reason for matching the CTEis to minimize distortion of the parts when temperature changes occur.Temperature changes may involve shifts by up to tens of degrees.Distortions occur because one component expands proportionately more orless than the second component. The distortion is undesirable and can beminimized by minimizing the CTE variation between the components.Mismatches of less than 60 ppb/° C. and preferably less than 15 ppb/° C.are achievable.

To address the issue of matching the thermal coefficient of expansionbetween the honeycomb article and a glass member, it is highly desirableto use a glass for the honeycomb article having a very low CTE. Such aglass is known and marketed by Corning Incorporated under the tradenameULE™. This silicate glass contains between about 6 to 8 wt % TiO₂ with apreferred content of about 7 wt % TiO₂. Contents outside of these valuescan be made in order to tailor the CTE to other desirable values aswell.

In order to create light weight honeycomb structures the presentinvention is designed to limit the amount of glass in the honeycombstructure. This is achieved by minimizing the thickness of the walls ofthe channels and thereby maximizing the channel openings within thehoneycomb structure. The extent to which this is achieved is defined bythe equation that relates area of the openings (OA) to the area of thetotal cross-section (TCA) of the honeycomb. Therefore, the open frontalarea (OFA) is equal to OA/TCA. In the current invention OFA up to 0.995are achievable. Depending on the ultimate use values of 0.5 to 0.995 arepreferred. For light-weight support structures 0.9 to 0.995 arepreferred. As noted supra, typical commercial OFA values up to 0.87 areobtainable. To achieve values above 0.9 additional process steps may berequired. For example, after sintering, either remove selected walls ofthe channels by machining (e.g., core drilling, water jet milling or bythinning the walls by grinding and polishing) or by chemical etching areuseful techniques for increasing the OFA value. Other machining optionsshould also work for those skilled in the art. It is also possible tochemically etch the channel walls to increase the OFA value. Anothertechnique to increase OFA is found in U.S. Pat. No. 5,458,834 (Faber etal.) assigned to Coming Inc. for an extrusion osmotic drying technique.

It is another embodiment of the present invention to utilize multiplehoneycomb articles for structural support of objects such as mirrors. Itis expected that multiple honeycomb units would be used primarily whendimensions begin to exceed about 10 inch diameter. The multiple unitscan be separately placed and act as individual islands of support.Alternatively, the individual honeycomb units can be bonded together toform a larger single structural support.

Various means of bonding a mirrored surface or backing plate to thehoneycomb structure exist (see U.S. Pat. No. 4,315,991, Hagy et al.) andinclude but are not limited to:

(1) glass to glass bonds at temperatures between 950° C. and 1600° C.This involves matching the contour of the honeycomb structure to thecontour of the glass plate. Secondly, the two structures are heatedtogether until fusion takes place between the two mated surfaces.

(2) machining the honeycomb structure to match the contour of the glasssurfaces. However, in this second embodiment, a second material isplaced between the two surfaces which on heating, will melt and form abond between the honeycomb structure and the plate. Usual temperaturesfor this process are about 1100° C.

Especially where the both ends of the honeycomb article are beingcapped, the cell walls of the honeycomb support may require a gaschannel to prevent the structure from undergoing large differentialpressure gradients between the channels and the ambient atmosphere.Several means of introducing gas into the channel are contemplated aspart of the present invention. Examples include, but are not limited to,(i) a structure which is not fired to complete density, thus having openconnected porosity between channels, (ii) holes placed in the channelsat any number of processing stages such as after extrusion but beforefiring or after firing etc., (iii) additional channel formers to theextrudable mix. Typical channel formers are starch or carbon black.These materials serve as inert material during the extrusion process,but would later be removed by combustion or oxidation.

In one preferred embodiment of the present invention an extrudedhoneycomb article is further processed to decrease its weight whilestill maintaining its ability to act as a structural support. In thisembodiment, the original honeycomb article having initially a uniformcircumference from one opposing end to the other, is fabricated toreduce the circumference at some point between the two opposing ends. Inthis manner the cross-sectional area of the opposing ends will begreater than that of the cross-sectional area at the longitudinalmidpoint of the honeycomb support. For a cylindrical support, this wouldtypically create a cylinder with a mid-longitudinal cross-section havinga smaller circular area than the opposing faces; while for a honeycombcolumn having a square cross-section, this would create amid-longitudinal cross-section in the shape of a rectangle (if twoopposing sides are trimmed) or a square (if all sides are trimmed). Ineither case the cross-section area at the mid-point of the longitudinalaxis will be smaller than the cross-sectional area of the opposingfaces.

Articles of this type are manufactured by first extruding to generate anarticle with uniform cross-section. Subsequent removal of material fromthe central portion, while leaving intact material on the top and thebottom, is then achieved by mechanical or ablative means. The article soformed, must retain its ability to bear a load on at least one of itsopposing faces. The designs of these structures will likely change withspecific applications, however circular- and rectangular-shaped opposingfaces are preferred. The upper face of the article is then bonded to theweight bearing load while the other face is used as a base to affix theweight bearing load and support to a frame or the like.

Since other modifications and changes varied to fit particular operatingrequirements and environments will be apparent to those skilled in theart, the invention is not considered limited to the examples chosen forpurposes of disclosure, and the present invention covers all changes andmodifications which do not constitute departures from the true spiritand scope of this invention.

Having thus described the invention, what is desired to be protected byLetters Patent is presented in the subsequently appended claims.

We claim:
 1. A process of preparing a glass honeycomb article comprisingthe steps of: a) providing an essentially homogeneous paste comprising avitreous silicate powder of high purity fused silica soot, a binder, alubricant, and a solvent, wherein said powder comprises particles ofless than about 0.4 micron; b) extruding strands of said paste through adie to form an intermediary article; c) drying said intermediary articlein order to remove residual solvent; d) debinding said intermediaryarticle in order to generate a brown honeycomb article; and e) firingsaid brown honeycomb article to partial or complete sintering thereof.2. The process of preparing a glass honeycomb article as recited inclaim 1, wherein said paste comprises 60 to 63 volume % of said silicatepowder.
 3. The process of preparing a glass honeycomb article as recitedin claim 1 wherein said steps (d) and (e) are performed in a singleheating regime.
 4. The process of preparing a glass honeycomb article asrecited in claim 1 farther comprising the step: f) heating saidintermediary article to a temperature between 850° and 1300° C. for aperiod greater than about 5 minutes in a chlorine of fluorine containingatmosphere for removing beta-OH and trace contaminants of alkali andalkaline earth elements, said step (f) being performed between saidsteps (d) and (e).
 5. The process of preparing a glass honeycomb articleas recited in claim 4 wherein said steps (d), (e) and (f) are performedin a single heating regime.
 6. The process of preparing a glasshoneycomb article as recited in claim 1 wherein said binder comprises anorganic polymeric substance.
 7. The process of preparing a glasshoneycomb article as recited in claim 1 wherein said lubricant comprisesorganic long chained fatty acids.
 8. The process of preparing a glasshoneycomb article as recited in claim 1 wherein said solvent comprisesaqueous organic solvent mixtures.
 9. The process of preparing a glasshoneycomb article as recited in claim 8, wherein said aqueous organicsolvent mixtures are water-alcohol or water-ester.
 10. The process ofpreparing a glass honeycomb article as recited in claim 1 wherein saidsolvent comprises essentially pure water.
 11. The process of preparing aglass honeycomb article as recited in claim 1 wherein said drying step(c) is performed using a dielectric oven.
 12. The process of preparing aglass honeycomb article as recited in claim 1 wherein said firing step(e) is performed using a heating regime comprising: i) loading saidbrown honeycomb article into a furnace and filling said furnace with aninert or reducing gas at room temperature; ii) heating said furnace atabout a 30° C./minute to about 1000° C.; iii) increasing the temperatureof said furnace at about 10-15° C./minute rate to about 1650° C.; iv)increasing the temperature of said furnace at about 2° C./minute toabout 1760° C.; v) holding the temperature of said furnace at about1760° C. for approximately 5-15 minutes; vi) cutting off power, allowingsaid article to cool at the cooling rate of said furnace to roomtemperature.
 13. The process of preparing a glass honeycomb article asrecited in claim 12 wherein said gas is selected from the groupconsisting of hydrogen, nitrogen, argon, helium, forming gas, carbondioxide and air.
 14. The process of preparing a glass honeycomb articleas recited in claim 12, wherein said glass honeycomb comprises highpurity fused silica, and further comprising the step: vii) holding atabout 1000° C. for approximately 5-15 minutes; said step (vii) beingperformed between said steps (ii) and (iii).
 15. The process ofpreparing a glass honeycomb article recited in claim 1, said processfurther comprising the steps: g) heat said glass honeycomb article to atemperature, sufficient to lower the glass viscosity to the point sothat the glass honeycomb article can be further processed in thefollowing step; and h) mechanically redraw said glass honeycomb articlein the longitudinal direction.
 16. The process of preparing a glasshoneycomb article recited in claim 15 wherein said step of mechanicallyredrawing further provides a torsional force around the longitudinalaxis of said redrawn honeycomb article creating channels that aretwisted in a helical array.
 17. The process of preparing a glasshoneycomb article recited in claim 15 wherein said step of mechanicallyredrawing further provides a bending force along the longitudinal axiscreating a U-shaped structure.
 18. A process of preparing a glasshoneycomb article comprising the steps of: a) providing an essentiallyhomogeneous paste comprising a vitreous silicate powder, a binder, alubricant, and a solvent, wherein said powder comprises particles ofless than about 0.4 micron; b) extruding strands of said paste through adie to form an intermediary article; c) drying said intermediary articlein order to remove residual solvent; d) debinding said intermediaryarticle in order to generate a brown honeycomb article; and e) firingsaid brown honeycomb article to partial or complete sintering thereof,wherein said debinding step (d) is performed using a heating regimecomprising: 20-200° C. @ 50° C./hr; 200-600° C. @ 20° C./hr; 600-900° C.@ 50° C./hr 900° C. for a six hour hold; 900-600° C. @ 500° C./hrcooling; and 600-20° C. @ cooling without power.
 19. The process ofpreparing a glass honeycomb article as recited in claim 18 furthercomprising performing the heating regime in an air of oxidizingenvironment to remove carbon residue.
 20. A process of preparing a glasshoneycomb article comprising the steps of: a) providing an essentiallyhomogeneous paste comprising a vitreous silicate powder, a binder, alubricant, and a solvent, wherein said powder comprises particles ofless than about 0.4 micron; b) extruding strands of said paste through adie to form an intermediary article; c) drying said intermediary articlein order to remove residual solvent; d) debinding said intermediaryarticle in order to generate a brown honeycomb article; and e) firingsaid brown honeycomb article to partial or complete sintering thereof,wherein said firing step (e) is performed using a heating regimecomprising: i) loading said brown honeycomb article into a furnace andapplying a vacuum at room temperature; ii) heating said furnace at abouta 30° C./minute to about 1000° C.; iii) increasing the temperature ofsaid furnace at about 10-15° C./minute rate to about 165° C.; iv)increasing the temperature of said furnace at about 2° C./minute toabout 1760° C.; v) holding the temperature of said furnace at about1760° C. for approximately 5-15 minutes; vi) backfilling said furnacewith an inert gas; and vii) allowing said article to cool at the coolingrate of said furnace to room temperature.
 21. The process of preparing aglass honeycomb article as recited in claim 20, wherein said glasshoneycomb comprises high purity fused silica and further comprising thestep: viii) holding at about 1000° C. for approximately 5-15 minutes,said step (viii) being performed between said steps (ii) and (iii).